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Re: [PATCH 09/11] pdp11 - example of a port not needing a speculation barrier
On July 27, 2018 3:27:49 PM GMT+02:00, Paul Koning <firstname.lastname@example.org> wrote:
>> On Jul 27, 2018, at 5:37 AM, Richard Earnshaw
>> This patch is intended as an example of all that is needed if the
>> target system doesn't support CPUs that have speculative execution.
>> I've chosen the pdp11 port on the basis that it's old enough that
>> is likely to be true for all existing implementations and that there
>> is also little chance of that changing in future!
>> * config/pdp11/pdp11.c (TARGET_HAVE_SPECULATION_SAFE_VALUE):
>> to speculation_safe_value_not_needed.
>> gcc/config/pdp11/pdp11.c | 3 +++
>> 1 file changed, 3 insertions(+)
>Correct, no speculative instruction now, and I don't think any of the
>people constructing PDP11s (yes there are some) are going to be adding
It's not really about speculative instructions but about things like having a branch predictor and ways to recover from badly predicted ones and thus from wrongly speculatively executed regular instructions.
Probably every pipelined CPU implementation nowadays has speculative execution.